The invention relates to a power semiconductor module having a baseplate on which at least one substrate is arranged which is fitted with power semiconductor chips and can be pressed via pressure elements against the baseplate.
When manufacturing power semiconductor modules, it is generally necessary to electrically insulate the power semiconductors or power semiconductor chips from the base body or heat sink which is used to dissipate heat. Ceramic isolators in disk form and with a metallized surface are widely used for this purpose. For example, direct copper bonding (DCB) substrates are used on one of whose surfaces the power semiconductor chips are essentially arranged, while the other surface is thermally coupled to the heat sink. The current-carrying capacities of the power switches required for higher-power applications are normally achieved by connecting individual power semiconductor chips in parallel to form a power switch. In addition, a number of power switches can be accommodated in a common can, thus simplifying the design from the assembly point of view by reducing the number of components, for example as is evident with regard to a three-phase bridge circuit for three-phase rectification or inversion, which may thus consist of only a single component instead of up to six controllable power switches and six non controllable power switches configured as discrete components. Examples of power semiconductor modules are widely known, see, for example, EP-A-0 265 833, EP 0 277 546B1.
Various methods are known for fabricating such power semiconductor modules.
By way of example, conductive connectors soldered onto the DCB substrate, in particular lugs, conductive connectors pressed onto the DCB substrate, in particular pressure contacts, and conductive connectors located in an additional insulator, for example a can frame, are used for electrical connection for the connections, to which conductive connectors the electrical connection is made for example by wire bonding, which is dependent on the insulator being fitted prior to this process.
With regard to thermal coupling to the cooling mediumxe2x80x94typically via the underneath of the componentxe2x80x94that surface of the DCB ceramic to which the chips are not fitted forms the underneath of the component in a module which has a DCB bottomxe2x80x94see DE35 21 572A1. As a rule, thermal coupling is achieved by pressing onto a typically metallic heatsink, from which heat is extracted by a cooling medium, such as air or cooling liquid, with the boundary surface between the DCB bottom and the heat sink advantageously having to be provided with an intermediate layer composed of a thermally linking medium, for example thermally conductive paste.
In a module having a bottom plate, that surface of the DCB ceramic to which the chips are not fitted is thermally coupled to a bottom plate which is generally composed of a metal or composite material; the coupling can be achieved by pressingxe2x80x94see DE41 31 200C2, DE41 11 247C2 and DE 41 22 428C2 typically, once again using a thermally linking medium, for example thermally conductive paste, by solderxe2x80x94see DE43 38 107C1 xe2x80x94or by other integral material joints, the latter in particular in the case of bottom plates composed of composite materials. The bottom plate for its part is then cooled which can be done, for example, by pressing against a heat sink or by applying a flow of a cooling medium, such as air or cooling liquid, subject to a suitable geometric configuration.
This prior art is subject to a number of disadvantages, as follows:
As stated above, power semiconductor modules were originally used to increase the integration level of power electronic circuit arrangements. If this aim is continued, it is necessary to. integrate power electronics together with peripheral functions such as actuation, intermediate circuit capacitors and the like in a controller close to the load: for a single-wheel drive using a three-phase motor, such a controller could be in such a form that it is placed against the motor housing, using the same cooling medium as the motor and, as electrical interfaces, could have a DC voltage feed and a bidirectional bus for information transmission. Power semiconductor modules according to the prior art in the described embodiments are not consistent with such greater integration since they are designed as enclosed units with a dedicated housing which, owing to requirements such as mechanical robustness, occupies more volume than would be necessary for greater integration; this fact is disadvantageous both from the point of view of costs and spatial economy and with regard to the electrical behavior, inter alia since it is influenced by parasitic supply line inductances; in addition, for example, accessibility to the electrical potentials that are present in the power semiconductor module is made more difficult through the housing, necessitating a not inconsiderable design cost for connection of the module.
The reliability of the electrical connections, particularly with soldered external connections, can leave something to be desired due to thermal fatigue. Efforts are being made to counteract this situation by suitable design of the connections, for example using expanding bars or special soldering methodsxe2x80x94see, for example, DD 283 236A5. On the other hand, if the electrical connections are produced using pressure contacts with metallic springs, then they can become soft, that is to say they can become increasingly plastically deformable, under the influence of the heat produced by the heat losses from the surrounding semiconductor chips, so that the contact pressure on the substrates will decrease. Furthermore, owing to their hardness, metallic alloys which are better for mechanical purposes often have a higher electrical resistance than materials optimized for good conduction behavior. Contact springs, which avoid these disadvantages, are generally expensive due to their particular material composition and are thus used little, for financial reasons. In any case, there is a risk that the ends of the contact springs or the contact surfaces opposite them will be ground away by scraping movements resulting from thermally dependent work by virtue of the design.
Ensuring the transfer of heat is associated with difficulties, involves complex design and is often not guaranteed over the required life of the power semiconductor module: a description of weaknesses and solutions proposed can be found, for example, in DE39 40 933 xe2x80x94Flexing of the soldered bottom platexe2x80x94,DE43 38 107C1 xe2x80x94Forming for a bottom platexe2x80x94,DE19 707 514A1 xe2x80x94Introduction of weak points in the bottom platexe2x80x94DE35 08 456A1 xe2x80x94Contact-pressure apparatus for the substrate by means of adjusting screws and spacersxe2x80x94or DE4 131 200 C2 xe2x80x94Exerting the pressure by spring elements, which may make adjustment necessary.
Power semiconductor modules constructed according to the prior art using the pressure-contact method are frequently distinguished by complex designs; for example, a design with a specially constructed bridge element is described in DE4 131 200C2. It can be seen that such a design must be matched to the specific circuits; circuit changes lead to tool costs with corresponding delivery times, which is disadvantageous with regard to more stringent requirements for prices and delivery times, even for new application-specific power semiconductor modules.
The object of the invention is thus to provide a power semiconductor module of the type mentioned initially whose design is reduced to the essential parts depending on the application and which can nevertheless be integrated better in a peripheral, for example, a controller. In the process, the greatest possible spatial economy and optimum accessibility to the electrical potentials together with thermal resistance of the connections should be ensured. Furthermore, the module to be designed should be capable of being manufactured economically using simple production processes and parts while still offering even greater flexibility for application-specific adaptations by using standard components.
This object is achieved according to the invention in that the pressure elements have elastic and conductive contact cords which are arranged between contact rails and the substrates.
Relatively strong pressure elements press the contact cords not only against these elements but also against the substrate metallizations arranged on the opposite side of the contact cords. This results in a reliable, long-life electrical connection between metallizations on the substrates, to which the semiconductor chips are electrically connected, and the pressure elements via the contact cords; furthermore, the contact cords press the substrates against the baseplate located on their opposite side, thus ensuring good heat transfer between the substrates, with the semiconductor chips to be cooled, and the cooled baseplate. According to the invention, the contact cords are in this case preferably provided with an elastic core and an electrically conductive sheath.
The arrangement and development of the contact cords according to the invention at the same time provides the precondition for an advantageous development of the overall design of the power semiconductor module according to the invention in the direction of comparatively simple manufacture, even for different application and operational fields. This is distinguished by a frame which defines fields being fixed by means of centering elements on the baseplate, with at least some of the fields accommodating corresponding substrates, which are fitted with power semiconductor chips.
This embodiment of a power semiconductor module according to the invention presets defined positions for the substrates with the semiconductor chips located on them. To match different requirements, it is possible to fill all the fields or individual fields with substrates, and then to make contact with them. There is therefore no need to change the tools for production of the frame for different applications. The same applies to the contact rails which are also provided according to the invention which, on the one hand, act as conductors for the current which flows via the substrates fitted with semiconductor chips and, on the other hand, act as force-transmitting elements. Predefinition of the arrangement via the centering elements which, according to another feature of the invention, are threaded bolts provided in the baseplate, results in the individual parts being reliably guided during the production process.
This is distinguished by the fact that opposing rails with recesses for the pressure elements which are formed as contact rails are provided on mutually opposite sides of the frame, and by the fact that the opposing rails are arranged on the centering elements. This embodiment according to the invention on the one hand clearly defines the arrangement of these opposing rails sincexe2x80x94provided with corresponding recessesxe2x80x94they are simply plugged onto the centering elements. For their part, the opposing rails have recesses for holding the contact rails, which can then be arranged at right angles to them in the main plane of the baseplate. All the said parts are thus matched to one another so that correct contact is made with the power semiconductor chips by means of the contact cords of the inserted contact rails.
As a continuation of the basic idea of the invention, the contact with the chips and the overall production of the power semiconductor module according to the invention is improved if pressure clips are provided on the centering elements and on the contact rails and opposing rails. The pressure clips can approximately match the opposing rails, in terms of length and width. The required contact pressure can thus be applied to the contact rails via the pressure clips by screwing nuts onto the screws or threaded bolts which are used as centering elements.
A further advantageous refinement of the basic idea of the invention is for the frame to be in the form of a grid and for the grid of the frame to have guides, formed integrally with it, for auxiliary contacts. These guides can thus be produced in one operation together with the frame, in particular in the course of injection molding. Electrical contacts can be inserted into the guides and, for example, produce a connection to a printed circuit board which is arranged on the power semiconductor module according to the invention and, for its part, forms an interface to an additional appliance, for example a controller.